1. Field of the Invention
This invention relates to a termination network for clamping line signals to a low and a high logic state and, in particular, to such a network which comprises an NPN and PNP transistor pair that are biased so as to selectively turn on when the line signal fluctuates below the low state and above the high state.
2. Description of the Prior Art
In electronic circuitry comprised of several discrete integrated circuits which are interconnected by conductors, it is important that the circuitry be capable of providing predictable and constant logic signals under all adverse conditions so as to provide a sufficient worst case level of noise immunity. However, even though care is exercised in fabricating and designing the circuits themselves, many times the received logic signal is not representative of that transmitted due to electrical noise and to the electrical parameters of the conductor itself. In general, there will be a mismatch on both the transmitting and the receiving ends of the conductor, causing reflections, which superimpose as noise, on the date being transmitted. For example, noise may superimpose spikes on the logic signal and inductance inherently associated with the actual conductor may introduce ringing oscillations on the logic signal. If the amplitude of the noise spike or ring is appreciable with respect to the amplitude of the logic signal, then the receiving logic may interpret each spike or ring as a logic pulse. Generally, incorrect interpretations cause malfunctions in the operation of the host system which could be of catastrophic proportions. The probabilities of malfunction are substantially increased when emitter-coupled logic (ECL) is used since ECL logic pulses have only about a one volt peak-to-peak amplitude as well as relatively high slew rates between the high and low states. In addition, the detrimental effect of randon noise pulses superimposed on transmitted data increases with an increase in the data rate of the transmission. At the present state of the art, ECL circuitry provides the fastest means for data transmission.
In the prior art, termination networks have been used near the receiving end of a data path to remove the noise and ringing effects from appearing at the receiver. In one terminating system, a network is connected to the line conductor. The network includes a pair of Schottky diodes, an inverter gate, a load resistor and two bias sources. One of the bias sources is ground and the other one is a negative voltage. The Schottky diodes are connected in parallel in a back-to-back arrangement between the line conductor and the inverter gate. The input and the output of the inverter gate are connected together. The resistor is connected between the output-input feedback of the inverter gate and the negative bias source. This arrangement sets a reference voltage at the center of the ECL logic swing (center between logic 1 and logic 0). Due to the fact that a Schottky diode base-emitter forward voltage equals approximately half the ECL logic swing, the network operatively clamps the voltages of the logic signals appearing on the line conductor which are above or below the base-emitter forward voltage of the Schottky diodes, i.e., above and below the logic 1 and logic 0 state, respectively. However, several problems have been found in the use of such a network. First, if the dc level of the high state of the line signal is above the threshold of one of the Schottky diodes, the signal remains clamped for the duration of the high state, thus causing a large and constant current through the load resistor. This load resistor, therefore, needs to be dimensioned such that it will sink the worst-case current supplied through the terminals. The requirement for this relatively high stand-by current is the major drawback of this implementation. Second, the threshold voltage of a Schottky diode varies substantially with changes in temperature. For example, at room temperature a Schottky diode may have a 0.5 volt threshold whereas at colder temperatures the threshold may be 0.7 volts, and at hot temperatures the threshold may be 0.3 volts. Accordingly, the clamping signal would vary with temperature from between 0.6 volts to 1.4 volts peak to peak. Thus, in cold environments ringing pulses having an amplitude comparable to that of the logic signal levels could be conducted into the receiver and in hot environments excessive current could be drawn through the load resistor. Third, since two Schottky diodes are required for each data line and a resistor and an inverter gate are required to set the threshold voltage, the expense of such a network for terminating a multiplicity of data lines would be substantial.